One-Step Microstructuring of TiO2 and Ag-TiO2 Films by Continuous Wave Laser Processing in the UV and Visible Ranges

The transformation of three kinds of mesoporous films of amorphous TiO2 under exposure to continuous wave (CW) lasers emitting at UV and visible wavelengths is investigated. Silver-free films, films impregnated with a silver salt, and films loaded with silver nanoparticles are considered. The silver containing films had previously been shown to exhibit a photochromic behavior. Intensity thresholds leading to laser-induced crystallization are evaluated using a combination of Raman spectroscopy, atomic force microscopy, and high-resolution electron microscopy. Crystallization in the anatase and rutile phases as well as ablation of the films are shown at 244 nm wavelength for increasing intensity, whatever the nature of the films. However, under visible light exposure, crystallization only occurs in the silver containing films. The oxidized-silver containing films, that are shown to be as transparent as the silver-free films in the visible range, appear to crystallize under visible light. The temperature rise occurring during visible light exposure and due to the plasmon-induced nanoparticle heating is measured. Finally, such a CW laser-induced crystallization is proven to be an accurate technique to produce permanent patterns with submicrometer widths and nanometer depths that can be tuned to within only a few nanometers. The patterns resist to temperature rises of 1000 degrees C.